1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention is directed to the parallel discharging of at least two alkali metal electrochemical cells powering an implantable medical device, such as a cardiac defibrillator, to increase the total discharge efficiency of the cells.
2. Prior Art
Early ventricular cardiac defibrillators used two lithium batteries, in series, as their power source. Due to the progress of new circuit designs, the electronic circuits in current generations of defibrillators now consume less energy than the earlier models and that makes it possible to use a single lithium battery to power the medical device. With a one cell design, however, the requirement for high current pulse capability, or power density, is even greater due to the lowered pulsing capacity of the single cell. Large electrode surface area is thus needed to accomplish this requirement. In general, when the electrode surface area is increased, more inert materials (current collector, separator, etc.) are incorporated into the cell which decreases the cell's volumetric capacity. Therefore, one of the concerns in a single cell design is the longevity of the medical device which is directly dependent on the cell's capacity and power efficiency.
While one approach to increasing the service life of an implantable medical device is to utilize a larger capacity cell, that is not desirable due to the current trend of reducing the size of implantable medical devices. Increasing the cell's utilization efficiency is a better choice. Lithium/silver vanadium oxide (Li/SVO) batteries have long been used to power cardiac defibrillators and the like. In general, when Li/SVO cells are accelerated pulse discharged, only about 80% of cathode efficiency is achieved at a 1.5 V cut off. In theory, about 20% of the SVO cathode capacity is wasted since the 1.5 V cut off is the voltage level at which the battery no longer possesses enough capacity to provide the required energy for high current pulse discharge within the preset period of time. Consequently, for the prior art single Li/SVO cell defibrillator design, a relatively large cell is needed to achieve an acceptably long device life, but that approach is in conflict with current trends of reducing the size of the medical device.
Therefore, there is a need to increase the utilization efficiency of batteries powering implantable medical devices by using the remaining 20% of the SVO cathode capacity while at the same time maintaining the high volumetric capacity of that cathode chemistry. One way to achieve this is to use a double cell design concept according to the present invention. The present double cell design can be housed inside the battery compartment of current implantable medical devices intended to be powered by a single cell, which further enhances the attractiveness of such medical devices.